Present invention embodiments relate to measuring parameters of respiratory mechanics, and more specifically, to techniques for measuring respiratory resistance, compliance, and inertance using an airflow perturbation device (APD).
The respiratory system at rest is dominated by two physical components: resistance and compliance. Resistance is the relationship between pressure and flow rate, compliance is the relationship between lung volume and pressure. Both of these components are useful for disease detection, and they may be related. In normal lungs, higher values of resistance usually accompany lower values of compliance, in diseased lungs, both may increase together, although this is not always the case.
Airflow perturbation devices were developed to measure resistance. An APD periodically inserts a resistance in the flow pathway from the mouth. Typically, this is done using a rotating wheel with open and closed segments. As the wheel rotates, changes (or perturbations) occur in both mouth pressure and respiratory flow. The depths of these perturbations depend on the respiratory resistance of the person breathing through the APD and the resistance of the APD itself.
Primary measurements made by the APD are mouth pressure, measured at the location of the APD closest to the mouth, and respiratory airflow, measured with a flowmeter in the flow pathway. The mouth pressure measurement represents the difference in pressure between the mouth and the atmosphere. If the side of the APD distal to the mouth is open to the atmosphere, the mouth pressure measurement divided by the flow measurement represents the resistance of the APD. Thus, APD resistance is known and respiratory resistance of the person using the APD can be inferred. Resistance of the respiratory system may be obtained noninvasively by dividing the mouth pressure perturbation magnitude by the flow perturbation magnitude.
Measurements with the APD have been confirmed to be those of respiratory resistances, consisting of the airways, lung tissue, and chest wall resistance components. APD resistance measurements have been compared to resistance measurements made with the body plethysmograph, forced oscillation, and esophageal balloon.
However, conventional APD techniques do not provide a measurement of compliance. Compliance depends on volume, whereas resistance depends on flow. If the flow rate perturbation is sinusoidal, then the corresponding volume perturbation may in principal be obtained by integrating sinusoidal flow. The result is a cosine function 90° out-of-phase with the sinusoidal flow. This is the manner in which resistance and compliance are obtained using the forced oscillation (FO) method of pulmonary testing and its variants, forced random noise (FRN) and impulse oscillometry (IOS). But compliance cannot be obtained in this way with the APD. The APD perturbation is not completely sinusoidal, nor does it need to be sinusoidal. Moreover, the APD makes measurements in the time domain with time series data, but the detection of phase angles requires frequency domain measurements based on complete sine waves. The APD mouth pressure and flow measurements are always in phase because all APD elements between the mouth and the APD opening exposed to the atmosphere are resistive in nature. These characteristics provide the APD with rapid response and the ability to separate resistance during the inhalation phase of breathing from resistance during the exhalation phase, but render the APD poorly suited for measuring a phase difference between pressure and flow to determine compliance.